The present invention relates to an impulse noise detector useful in, for example, a car radio.
Impulse noise such as ignition noise and mirror noise occurs frequently in the car environment, and is readily picked up by the antenna of a car radio. To prevent such impulse noise from becoming audible, a car radio requires an impulse noise reducer.
FIG. 1 is a block diagram of a frequency-modulation (FM) stereo car radio comprising an antenna 1, a tuner 2, an FM demodulator (DEMOD) 3, an impulse noise reducer 4, an impulse noise detector 5, an FM stereo demodulator 6, and a pair of loudspeakers 7 (only one of which is visible). An FM radio-frequency broadcast signal received by the antenna 1 is selected and amplified by the tuner 2, then demodulated by the FM demodulator 3 to obtain an FM composite signal. The FM composite signal includes a left-right sum component with audio frequencies up to fifteen kilohertz (15 kHz), a 19-kHz pilot component, and a left-right difference component amplitude-modulated around a 38-kHz carrier frequency, with an upper sideband extending to 53 kHz. Impulse noise picked up by the antenna 1 may also be present, but this noise is detected by the impulse noise detector 5 and blanked out by the impulse noise reducer 4. Further demodulation of the FM composite signal by the FM stereo demodulator 6 produces right-channel and left-channel audio signals, which are reproduced through the loudspeakers 7.
FIG. 2 shows the internal structure of an impulse noise reducer 4 and an impulse noise detector 5 employed in the prior art. The FM composite signal is received at an analog input terminal 8. The impulse noise detector 5 comprises a high-pass filter (HPF) 9, an amplifier (AMP) 10, an automatic-gain-control (AGC) circuit 11, and a monostable multivibrator 12. The impulse noise reducer 4 comprises a gate 13. The high-pass filter 9 has a cutoff frequency high enough to reject all of the above-mentioned components of the FM composite signal. Impulse noise includes still higher frequency components that are not rejected. The output of the high-pass filter 9, which comprises impulse noise and other noise, is amplified by the amplifier 10, the gain of which is controlled by the AGC circuit 11 so as to keep the amplified noise signal below the threshold of the monostable multivibrator 12. The AGC circuit 11 is designed to respond slowly to changes in the noise level, however. When impulse noise occurs, the monostable multivibrator 12 cannot track the rapid rise in the noise level, so the output of the amplifier 10 triggers the monostable multivibrator 12, which outputs an impulse noise detection signal to the gate 13. The output of the gate 13 is held constant while the impulse noise detection signal is active, so impulse noise is effectively suppressed.
One problem in this prior-art circuit is that the monostable multivibrator 12 is triggered not only by impulse noise, which typically lasts only a few tens or a few hundreds of microseconds, but also by tone-burst signals, in which a sine-wave signal lasting several milliseconds is preceded and followed by a signal with zero amplitude. The result is the unwanted blanking of these tone bursts.
Another problem is that intermodulation distortion can cause some of the energy of the composite FM signal to leak into frequency bands above 53 kHz, affecting the operation of the AGC circuit 11 and making accurate detection of impulse noise difficult.
It is accordingly an object of the present invention to detect impulse noise lasting for brief intervals of time, while avoiding the detection of signals that appear abruptly but last for longer intervals.
Another object of the invention is to detect impulse noise accurately in the presence of leakage of audio signal components caused, for example, by intermodulation.
The invented method of detecting impulse noise comprises the following steps:
high-pass or bandpass filtering of an input signal;
detecting an envelope of the filtered signal;
calculating a first average value of at least some of the values of the envelope signal over a predetermined time interval;
multiplying the first average value by a first factor to obtain a threshold value;
comparing at least one value of the envelope signal in the predetermined time interval with the threshold value;
activating an impulse noise detection signal if the compared value exceeds the threshold value; and
de-activating the impulse noise detection signal if the compared value does not exceed the threshold value.
In one aspect of the invention, all values of the envelope signal in the predetermined time interval are used in calculating the first average value.
In another aspect of the invention, the values used in calculating the first average value are those that do not exceed the product of a second average of all of the values in the predetermined time interval, multiplied by a second factor.
In another aspect of the invention, the envelope signal is obtained by detecting a preliminary envelope, taking differences of values in the preliminary envelope, then detecting the envelope of the difference values.
In another aspect of the invention, the steps described above are performed twice, using bandpass filters with different passbands, and the two resulting impulse noise detection signals are combined in different ways, depending on the received level of a radio-frequency signal from which the input signal is obtained, to generate an output impulse noise detection signal.
In another aspect of the invention, the first factor is adjusted according to the received level of the radio-frequency signal.
The invention also provides impulse noise detectors and a noise reduction system employing the invented method.
By using a multiple of the envelope signal averaged over a time interval as a detection threshold, the invented method detects short-duration impulse noise without detecting abruptly rising signals of longer duration.
By taking an envelope of differences of a preliminary envelope, the invented method emphasizes impulse noise, relative to intermodulation products and other audio leakage, so that the impulse noise can be accurately detected.